In a cellular wireless system, a service area is divided into a number of coverage zones generally referred to as cells. Wireless terminals in a cell communicate with the base station that serves the cell. Wireless terminals may include a wide range of mobile devices including, e.g., cell phones and other mobile transmitters such as personal data assistants with wireless modems.
A problem with known cellular communications systems is that transmission by wireless devices in one cell may collide with transmissions by wireless devices in a neighboring cell. For example, OFDM takes a given bandwidth and splits it into a number of evenly spaced tones that can be used to transmit data. When transmissions by devices in overlapping cells use the same tone or set of tones, multiple collisions may occur over a period of time due to the operation of devices in neighboring cells. This problem is particularly noticeable where transmissions are periodic or nearly periodic.
In periodic or nearly periodic situations, mutual interference caused by wireless terminals in adjacent cells may be highly correlated. This is because when a tone assigned to a wireless terminal A corresponding to a first base station is the same as a tone of another wireless terminal B corresponding to an adjacent base station, in the next transmission period, the tone of wireless terminal A will again be the same as wireless terminal B in the case where the tones are assigned using the same function and recur periodically. Correlated interference of this type can cause signals transmitted by the same two wireless terminals to repeatedly interfere with each other over a long period of time. If the two interfering wireless terminals are disadvantageously located, the base stations in the overlapping cells may not be able to detect the signals correctly from the two interfering wireless terminals for a long period of time.
In order to reduce the risk of correlated or prolonged interference it would be beneficial if it was possible to assign tones to devices in neighboring cells in a manner that would minimize the risk of correlated interference. Unfortunately, this goal is complicated by the difficulties associated with trying to synchronize transmission from neighboring base stations.
One reason different cells are not synchronized is because of cost considerations concerning hardware and/or software implementation. For example, GPS (Global Positioning System) may be needed in the base station of each cell if accurate synchronization between base stations is desired.
The difficulty of synchronizing OFDM signals is a function of the carrier frequency used and the tone spacing. OFDM can use a carrier frequency of, e.g., 1.9 GHz and the space between each tone be around, e.g., 10 KHz. In such system the spacing between tones is relatively small in comparison to the carrier frequency. As a result, even relatively small differences in the carrier frequencies used by neighboring base stations due to oscillator errors in neighboring base stations may result in differences in tones of neighboring base stations by being off by the spacing of a full tone or more. For example, a first base station trying to transmit tone 0 at 1.9 GHZ may actual transit it at 1.9 GHz+10 KHz causing tone 0 of the first base station to be transmitted at the frequency of tone 1 of a neighboring base station which properly transmits tone 0 at 1.9 GHz and tone 1 at 1.9 GHz+10 KHz. Given the difficulty in synchronizing base stations in neighboring cells, the transmitter of neighboring cells are frequently unsynchronized.
The unsynchronized nature of different cells complicates the problem of minimizing the interference of wireless terminals between cells so that repeated interference encountered by any individual device is minimized.
In view of the above discussion, it becomes apparent that there is a need for minimizing the potential for collisions between transmissions that occur in neighboring cells of a wireless communications system, particularly in the case of unsynchronized base stations. It is desirable that the probability that transmissions from any given device in neighboring cells will collide repeatedly be minimized to avoid extended periods where communication signals are blocked for any particular device.